Conventionally alumina is obtained by firing aluminum hydroxide, but aluminum hydroxide produced by the Bayer process usually contains 0.3-0.5% NA.sub.2 O and accordingly alumina obtained from this aluminum hydroxide contains 0.4-0.5% Na.sub.2 O. The alumina is produced by dehydrating aluminum hydroxide, followed by granulation, curing, drying and firing at high temperatures of 900.degree.-1110.degree. C. When the Na.sub.2 O content is too high, the strength of the alumina as a catalyst carrier drops. For instance, when an auto converter filled with such a carrier which carries a catalyst metal is used, extended vibration causes the carrier to wear and be pulverized and at the same time the efficiency of the catalyst metal as a catalyst deteriorates. For this reason, it is desirable that as little Na.sub.2 O as possible be present in the alumina used as the catalyst carrier. The typical known processes developed for this purpose are as follows:
(1) Aluminum hydroxide is partially dehydrated by roasting, and then it is agglomerated by mixing it with water; and the alumina agglomerate obtained is flushed with water (Pat. Pub. SHO 45-40168).
(2) Aluminum hydroxide is roasted at 500.degree.-600.degree. C. and then flushed with an aqueous solution of phenol (Pat. Pub. SHO 34-10620).
(3) Aluminum hydroxide is agitated together with 0.1-5% ammonia solution or an aqueous solution of ammonium salt at 110.degree.-250.degree. C. in an autoclave (Pat. Pub. SHO 32-630).
(4) Alumina or aluminum hydroxide is mixed with calcium chloride; heated at over 800.degree. C.; and then flushed with water or an aqueous solution of hydrochloric acid (Pat. Pub. SHO 34-10621).
(5) Aluminum hydroxide wetted with hydrochloric acid or an aqueous solution of aluminum chloride is roasted within a vessel fabricated of silicious refractory (Pat. Pub. SHO 47-5744).
(6) Aluminum hydroxide is granulated (or agglomerated), roasted at 426.degree.-566.degree. C. and then washed with a dilute acid (U.S. Pat. No. 2,769,688).
(7) Aluminum hydroxide and an aqueous solution of an acid are reacted to yield boehmite at over 177.degree. C. in an autoclave, followed by dehydration and flushing with water (U.S. Pat. No. 2,774,744; U.S. Pat. No. 3,628,914).
(8) Aluminum hydroxide is roasted at 250.degree.-500.degree. C. for 2-6 hours; flushed with a dilute acid solution at room temperature--100.degree. C. and then filtered, followed by drying and firing (U.S. Pat. No. 3,112,279).
In the above processes (1)-(4) and (6)-(8) aluminum hydroxide is roasted and then flushed with a dilute acid; or treated with a dilute acid in an autoclave followed by flushing and filtering, thereby yielding low-soda alumina.
A considerable amount of flush water is consumed in a flushing process for each of these manufacturing methods and the disposal of waste liquid generated in the flushing process is costly; in process (5) there is no flushing process but acid is employed and preventing the acid and acid decompositions from being diffused into the atmosphere is quite expensive.